(a). Field of the Invention
The present invention relates to a power Off upshift control method for an automatic transmission, and more particularly, to a control method for an automatic transmission that prevents under-shooting and tie-up while performing power Off upshift control.
(b). Description of the Related Art
Shifting is generally performed in an automatic transmission in accordance with changes in throttle valve opening. That is, throttle valve opening is varied through driver manipulation of the accelerator pedal, and such changes in throttle valve opening correspond to different locations on a shift pattern such that shifting is performed.
The different types of shift control include power On/Off upshifting and power On/Off downshifting. Power On upshifting and power On downshifting refer to shifting taking place when the driver depresses the accelerator pedal, while power Off upshifting and downshifting refer to shifting when the accelerator pedal is released.
If the accelerator pedal is released in a state approaching or at full throttle valve opening, lift-foot-up shifting is effected in which an upshift line on the shift pattern is crossed such that shifting into a higher speed is performed. That is, with reference to FIG. 1, if throttle valve opening is changed from 100% to 0% (i.e., if the accelerator pedal is released by the driver after being fully depressed), hydraulic pressure control occurs such that a state as close to neutral as possible is realized. At this time, a drop in engine rpm NE1 occurs as engine rpm NE1 come close to corresponding to turbine rpm NT1, and if a shift finish point is reached, duty control is completed such that hydraulic pressure is increased, thereby realizing upshifting.
In application, however, during power Off upshifting, both changes in engine rpm drop characteristics (according vehicle driving state and load) occur, and clutch timing and solenoid valve quality problems arise. With regard to the change in engine rpm drop characteristics, this causes a change also in the speed at which turbine rpm decrease. If such reductions occur too quickly (before the shift finish point) under shooting occurs, while if the reductions in rpm occur too slowly (after the shift finish point) tie-up occurs. The problems of under shooting and tie-up worsen as a result of either too slow or fast hydraulic pressure response times.
In the prior art control method as described above, during lift-foot-up shifting, as a result of tie-up and under shooting, which are caused by a reduction in turbine rpm occurring because of a drop in engine rpm, trouble in shift synchronization occurs. This results in the generation of shift shock, reducing ride comfort.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a power Off upshift control method for an automatic transmission which varies a shift finish point according to changes in turbine rpm during power Off upshifting such that shifting occurs always at the shift finish point.
To achieve the above object, the present invention provides a power Off upshift control method for an automatic transmission comprising the steps of determining, during an upshift operation, if power Off upshifting is taking place; determining if power Off upshift conditions are satisfied; measuring a hydraulic pressure response time, establishing a maximum duty value for preventing forced shifting, and calculating the hydraulic pressure response time when the maximum duty value changes to 100% duty, these processes being performed if the power Off upshift conditions are satisfied; calculating a shift finish point after the hydraulic pressure response time is calculated; determining if the shift finish point corresponds to an actual shift finish point; and setting the maximum duty value at 100% duty if the shift finish point corresponds to the actual shift finish point.
According to a feature of the present invention, the power Off upshift conditions include a throttle valve opening less than or equal to 0.65V or an idle switch being in an On state, and an ATF temperature being greater than or equal to 0xc2x0 C.
According to another feature of the present invention, if the power Off upshift conditions are not satisfied, the upshift control method is placed in a normal operating mode such that the shift finish point is not altered according to turbine rpm.
According to yet another feature of the present invention, the shift finish point is calculated by detecting turbine rpm Nt for a predetermined number of cycles to obtain an average turbine rpm rate of change NTAV; calculating a target turbine rpm NTob, the target turbine rpm NTob being calculated by multiplying No by a target gear ratio (NTob=Noxc3x97target gear ratio), where No is a turbine input rpm Nt divided by the gear ratio; and determining the shift finish point by multiplying the average turbine rpm NTAV by the hydraulic pressure response time TPR plus the target turbine rpm NTob [i.e., (NTAVxc3x97TPR)+NTob=SFP].
According to still yet another feature of the present invention, the average turbine rpm rate of change NTAV is obtained by detecting turbine rpm Nt for each cycle (20 ms) for four cycles to determine a rate of change of turbine rpm (NT1-NT4), then dividing this rate of change of turbine rpm (NT1-NT4) by four cycles to obtain the average turbine rpm rate of change NTAV.